There is a need to detect the presence of radioactive materials so as to prevent unauthorised passage of such materials across national borders, or into regions in which such materials are prohibited. A suitable method would be capable of performing the detection as a vehicle passed through a detection zone, preferably without stopping in the zone, so as not to excessively impede the flow of traffic. Thus the method would preferably be capable of detecting the presence of such materials in a period of about 10 seconds or less. The method should preferably have high accuracy, i.e. a low level of both false positives (signalling an alarm when no suspect material is present) and false negatives (failing to detect the presence of suspect material).
Such detection is complicated by the fact that acceptable materials may emit a certain level of ionising radiation, for example due to the presence of elevated concentrations of naturally occurring radioactive materials (NORMs), or of legitimate radiopharmaceutical products etc. Some existing systems, which use simple plastic scintillation detectors, measure only the gross level of radiation, in the form of gamma-rays, emitted by a target. Such systems are prone to a high rate of false positives if the threshold level of radiation detection is set too low or a high rate of false negatives if the threshold level of radiation detection is set too high. Such systems are unable to distinguish legitimately traded goods containing elevated concentrations of NORMs from illicit or inadvertent and unlicensed goods containing radioactive materials.
A second generation of detectors sought to measure the gamma ray spectrum of the target and compare that spectrum to the spectra of known controlled or other radioactive materials of interest. Such detectors, commonly referred to as spectroscopic detectors, identify the isotopes through the presence of specific gamma ray lines (energies) being present in a measured gamma-ray spectrum. High resolution spectroscopic equipment of this type is very expensive and is subject to poor reliability in field deployment due to the challenging operating conditions. Lower resolution spectroscopic equipment is less expensive but is subject to poorer performance with respect to correct isotope identification leading to high rates of both false positive and false negative alarm rates.